Facebook certainly likes to at least have sense to know what you are conversating about. Sometime in 2016 we and my buddy abroad got our accounts frozen "due to security reasons" at exact same time; what we were doing is having fun with FB Messenger and sending each other PGP-encrypted messages. This least about 2 months and my buddy is Egyptian, so I am pretty sure at some point FB said "we don't know what they chat about and enough is enough". I got my account recovered after multiple layers of verification including video-call to hold up my ID done by third-party ... my friend never gotten his reinstated.

Facebook definitely has some kind of chat monitoring and real-time censorship in place. For example, I once couldn't send a message in private chat if it included a link to one of the online weed stores. Remove the link, and it goes through just fine. Put the link there, and the thing just hangs and errors out with no coherent explanation.